LEDs with a vertical geometry are promising candidates for deployment in solid-state lighting products because they can handle the high drive currents needed to deliver a high luminous output. Manufacturing this form of LED requires a wafer-to-wafer bonding process,which involves many variables that need to be optimised for the specific device design, say Thomas Uhrmann, Eric Pabo,Viorel Dragoi and Thorsten Matthias from EV Group. White LEDs are already impacting the general lighting market, and their penetration in this sector is widely expected to rise. The rate of adoption will be governed by three factors: luminous efficiency, cost per lumen installed, and lumens per socket.
One way to improve all three areas simultaneously is to increase LED efficiency. But even greater gains to the lumen output of the luminaire and its cost-perlumen are possible by combining gains in efficiency with a higher drive current for the device. Cranking this up, however, increases LED heating. And to cope with this, the system designer must carefully manage heat that flows from the device junction to the package, fixture and surrounding environment.
It is possible to increase the rate that heat flows out of the LED by using metal wafer bonding for transfer of the epistructure to another substrate. Take this step and the LED benefits on two fronts: it can rapidly conduct heat away through a metal bond with a low thermal resistance, and it can dissipate heat through a substrate with low thermal resistance.
This approach can not only enhance the electrical properties of the nitride-based white LED, but also its blue variant and its red, orange and yellow cousins that are made from the AlInGaP material family. At EV Group, which is based in St. Florian, Austria, we are supporting the manufacturing of LEDs produced with a metal bonding process. Our involvement includes the recent launch of the first tool dedicated to this fabrication step – the EVG 560HBL. This piece of equipment is designed to deliver very high yields thanks to optimisation of pressure and temperature distributions, and it sets a new benchmark for throughput of up to 176 bondsper- hour for 2-inch wafer equivalents.
Manufacturing nitride LEDs with a metal bonding process presents some different challenges. Sapphire, the most widely used platform for making blue and white LEDs, has the desirable attribute of high transparency, but it is a poor heat conductor. Consequently, high-power LEDs employing a lateral design are poor at dissipating their heat and run hot, which degrades device performance. To combat this, some LED manufacturers have developed vertical LED designs, which involve substituting sapphire for another carrier with higher thermal conductivity.
Switching to this design also simplifies the manufacturing process by eliminating an etching step required to form the n-contact in a lateral LED. In addition, the vertical architecture produces a vertical current path, leading to a lower forward bias and eliminating current crowding issues that are frequently seen for other LED designs. And there are other benefits too: the addition of the metal bonding layer ensures that all of the light exits from the top of the LED; and manufacturing may be simplified, because the vertical LED design uses the same process flow for different die sizes.
In addition to high thermal conductivity, the bond interface in a vertical high-brightness LED must have excellent electrical conductivity. Fortunately, high thermal conductivity and high electrical conductivity tend to go hand-in-hand, and are found in germanium and metallic substrates. Both of these are popular, but silicon is emerging as a carrier material, featuring high heat dissipation and low thermal expansion. Using silicon also enables vertical LED producers to include a Zener diode directly into the carrier substrate, which serves for the electrostatic protection of the sensitive GaN LEDs.
The metal bonding approach is the only one that is applicable to high-brightness LEDs, due to the requirement for low thermal resistance. This is not the only benefit of this type of bond, however – it can also increase the luminous efficiency of the device. It was first used in AlInGaP-based LEDs that are grown on GaAs substrates. Spontaneous emission from these devices is assumed to be isotropic, with half of all the light generated propagating towards the substrate, where most of it is absorbed, leading to lowering of overall device efficiency. Inserting a distributed Bragg reflector beneath the light-generating region of the LED could prevent this light loss to the substrate, but in practice this only works effectively on one optimised direction of light emission, and it is better to turn to wafer bonding, where a reflective layer is included in the metal stack.
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